The filter of the invention is used for the removal of liquid from suspensions, sludges, and the like, particularly sludges derived from thinner suspensions by thickening. Belt filters for this purpose have been described in German Patent Specification No. 1,277,203 which shows a feed device for the sludge and a power-driven, traveling, endless-screening belt guided over rollers and made of wire mesh or plastic mesh. The lower run of a pressure belt, also guided on rollers and situated above the screening belt, runs in the opposite direction to the substantially horizontal upper run of the latter; the pressure belt extends over part of the length of the upper run of the screening belt, is pressed against the latter as it travels, the pressure forces increasing along the path of contact. Belt filters of this kind can be operated continuously and, if necessary, without supervision; both the capacity and the degree of dryness obtained in the press cake satisfy many requirements.
Another filtering system is described in Bulletin 700 of the firm of Smith & Loveless, U.S.A., in which sludge taken from an inspissator is passed on to a first traveling screening belt the end of which ejects partly-drained product to a second screening belt following it. As the second screening belt effectively forms a prolongation of the first, this system neither saves over-all length nor gives any special advantage unless it is that of subdividing an already known belt into two separate belts. A leaflet entitled "GUVA" published by the GUVA AG Verfahrenstechnik und Apparatebau, of Rumlang, Switzerland, describes a filter tower in which use is made of two generally vertical moving filter cloths which define between them a gap which narrows toward the bottom. Some materials to be filtered subject the filter cloths to the danger of becoming seriously clogged; when sludge is used which in the initial state is a thin liquid, this causes a comparatively high pressure in the vertical gap which prevents effective preliminary drainage and results in a turbid filtrate. The moisture to be extracted can emerge on both sides of the filter cloth, and the apparatus can be erected on a very small area. The height and space required, however, are considerable, and the constructional expense is on a level with that involved in the previously-described type of belt filter.
The German Pat. No. 881,969 suggests, for the withdrawal of liquid such as the juice from a pulp of fruit or the like, a number of systems equipped with traveling sponge belts, which by their own suctional effect serve to extract the moisture or juice from the material fed to the apparatus, the liquid being later pressed from the belt. In order that adequate lengths of these sponge belts can be accommodated, systems are also described in which a number of these traveling absorbent endless belts are mounted one above the other. A knowledge of these multiple traveling absorbent layers (mainly used for the removal of juice or water from fruit pulp or foodstuffs) has nevertheless not proved an inducement to the technician concerned with the drainage of clarification sludge, in the period of time that has passed since the publication of the aforementioned patent, to apply the same technique to this other sphere, which has different conditions.
It is, therefore, an outstanding object of the present invention to provide a belt filter which will occupy little space and yet provide ample active filter surface, so that high filter performance will be obtainable with only moderate space requirements and expenditure.